In a communication system, different categories of reference signals are used. One category of reference signals is used to estimate the channel which is needed to enable coherent demodulation of received signal containing both control and data information. Another category is used for channel quality measurements and enabling scheduling.
For example, in the 3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution) uplink, reference signals which are used for coherent demodulation are called demodulation reference signals (DRS) and reference signals which are used for channel quality measurements are called sounding reference signals (SRS).
Both SRS and DRS reference signals in LTE are decomposed of a cell specific base sequence with a predefined length and a corresponding cyclic shift in time domain. In LTE uplink, the base sequence is a Zadoff-Chu (ZC) or a Computer Generated (CG) sequence. The ZC sequence is used as the base sequence when the length of the base sequence is larger than 24 sub carriers. Otherwise Computer Generated (CG) sequence is used. These two kinds of sequences have the advantage of having constant amplitude in time and frequency domain, which is desired for uplink reference signal (RS). Cyclic shift is a linear phase rotation in frequency domain applied to the base sequence. This linear phase shift in frequency domain is equivalent to a cyclic shift in time domain. Different values of cyclic shifts are used to generate different orthogonal reference signals within one cell, from one base sequence.
In a multi user communication system, where several users with one transmit antenna transmit simultaneously in the same band, as in LTE uplink DRS corresponding to multi-user MIMO or LTE uplink SRS, one cyclic shift is used to generate reference signal of each user. As the reference signals corresponding to users are orthogonal, cyclic shift value and reference signal can be used to separate different users at the receiver side.
In the LTE uplink, predefined and distinct symbols in the uplink sub-frame are dedicated to transmit reference signals (RS), which is illustrated in FIG. 1. In the case of normal Cyclic Prefix (CP), DRS occupies the 4th symbol of each uplink slot and SRS is transmitted in the last symbol of some sub-frames which are configured by eNB. The transmission of DRS and SRS for each user equipment (UE) is independent. If one UE is scheduled in a number of resource blocks (RB) in a subframe for transmitting the physical uplink shared channel (PUSCH), this UE will transmit DRS with length equal to scheduled bandwidth on the 4th symbol of the two slots of the subframe. The transmission bandwidth and sub-frames for SRS is configured by eNB. Even if a UE is not scheduled for data or control transmission, it can still transmit the SRS in the last symbol of the second slot in the configured SRS transmission subframes.
For uplink DRS, multiple users use different cyclic shifts of the base sequence and transmit simultaneously only in the case of uplink multi-user MIMO transmission. Otherwise different users are separated by FDM and TDM. More precisely, when a single user is scheduled in a set of resource blocks and a subframe in LTE uplink, as there is only one transmit antenna, only one DRS is needed and consequently only one cyclic shift is used and remaining cyclic shifts are left unused. For SRS on the other hand, multiple users are often scheduled in the same time frequency resource and are separated through different cyclic shifts of the base sequence.
For DRS in the case of MU-MIMO and SRS, cyclic shifts must be allocated to different users in such a way that the corresponding time shift between two users becomes larger than the channel delay spread. Otherwise, channels corresponding to different users will interfere with each other when performing channel estimation at the receiver. Therefore, not all available cyclic shifts of the base sequence can be used in practice.
Assume the case of an LTE system operating at 5 MHz bandwidth with a typical urban (TU) channel model with UE speed 3 km/h. This is a typical wireless communication channel. According to LTE Release 8, there are a total number of eight cyclic shifts available for SRS and DRS, i.e. up to eight users can be code multiplexed together. If consecutive cyclic shifts are allocated in frequency domain to different users, time shift between users is about 32 samples which is smaller than TU channel delay spread (39 samples). Therefore, in order to give enough protection against multi-path for each user at the receiver, at most every other cyclic shift must be allocated to each user which means that only four out of eight users can be code multiplexed together under the assumption of a TU channel.
LTE-Advanced is the evolution of LTE where the user equipment (UE) or a relay node (RN) has up to four transmit antennas and it is important to know the channel from all transmitter antennas to the receiver antennas since this information is used to select modulation and coding scheme for the data transmission and also to select precoding matrix to be used for the transmission. A precoding matrix, taken from a codebook of available precoding matrices, is selected by the receiver to match the channel and improve the quality of the transmission. The data and the DRS are precoded by the same precoding matrix whereas the SRS are not precoded. Hence, one SRS needs to be transmitted from each of the UEs transmission antennas. Let us consider SRS allocation for this case. When user equipments or relay nodes with multiple antennas are supported, a larger number of cyclic shifts must be allocated per UE or RN compared to LTE to distinguish each antenna and each user. In this case, the available eight sounding sequences are rapidly exhausted.
The patent document WO 2009/017363 relates to a method and apparatus for multiplexing a reference signal from a User Equipment (UE), not having any other signal transmission in the respective Transmission Time Interval (TTI) with a reference signal from another UE also having data transmission in the respective TTI, or with the control signal and reference signal from another UE transmitted in the respective TTI. The multiplexed reference signal from the UE not having any other signal transmission in the respective TTI can serve as a sounding reference signal to enable the serving base station to apply link adaption to a subsequent signal transmitted by the UE or it can serve as a reference signal conveying state information, such as resource request or service request. In this document, only the problem of limited number of SRS has been addressed. Other problems still remain.